Kim BongSoo, Beebe Jeremy M, Jun Yongseok, Zhu X-Y, Frisbie C Daniel
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Am Chem Soc. 2006 Apr 19;128(15):4970-1. doi: 10.1021/ja0607990.
Understanding electron transport in metal-molecule-metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (-S) or isocyanide (-CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au-CN contact is more resistive than Au-S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the beta value = 0.5 A-1).
理解金属-分子-金属（MMM）结中的电子传输对于分子电子学的发展至关重要。决定MMM结电导率的关键因素包括金属-分子接触的性质和分子主链的电子结构。我们使用导电探针原子力显微镜（CP-AFM）和紫外光电子能谱（UPS）研究了具有硫醇（-S）或异氰化物（-CN）连接基的长度不断增加的刚性共轭低聚并苯的电子传输性质和价电子结构。我们发现，对于这些共轭体系，金-异氰化物接触比金-硫醇接触电阻更大。接触电阻的差异与UPS测量结果相关，该结果表明异氰化物系列的最高占据分子轨道（HOMO）的能量（相对于金的费米能级）低于硫醇系列的HOMO，这表明异氰化物连接的分子在接触处存在更高的隧穿势垒。相比之下，这两个系列分子的HOMO位置差异似乎并未影响结电阻的长度依赖性（即β值 = 0.5 Å-1）。
